Different forms of vitamin K:

• There are three forms of Vitamin K:
  • Phylloquinone (K1)- found in plants
  • Menaquinone (K2)- synthesized by bacteria in gut of human
  • Menadione (K3)- synthetic form of vit-K provided as medical supplement.
• All these has ubiquinone like structure.

Properties of vitamin K:

• Vitamin K1 is a yellow viscous oil whereas vitamin K2 is a yellowish crystalline solid.
• It is kept in dark bottles as it is sensitive to light.
• It is destroyed by irradiation, strong acids, alkalies and oxidizing agents.

Biological roles of Vitamin K:

1. Vitamin k acts as co-enzyme for enzyme carboxylase.

• It catalyzes the carboxylation of inactive precursor of blood clotting factor II, VII, IX and converts them into their active forms.
  • II – Prothrombin
  • VII – proconvertin
  • IX – Christmas factor or Anti-hemophilic factor B
  • X- stuart prower factor

• In-active form of these factors contain glutamate residue in their polypeptide chain. Vitamin k catalyzes the carboxylation of glutamate residues at  γ-position forming  γ-carboxyglutamate.
• Polypeptides containing  γ-carboxy glutamate acts as active factors.
• Warfarin and Dicumarol inhibit this carboxylation reaction and therefore act as anti-coagulant.
• Vitamin K also carbohydrates glutamate residues of osteocalcin, a protein found in bones and convert it into its active form.

2. Role of Vitamin K in blood clotting:

• Vitamin K convert glutamate to -carboxy glutamate.

•  γ-carboxy glutamate has two exposed negative charges.
• This  γ-carboxy glutamate containing blood clotting factor will interact with divalent cation calcium bound to the platelets cell membrane.
• The complex formed is called as prothrombin calcium complex or pro-convertin calcium complex etc.
• This sequence of events leads to the conversion of prothrombin to thrombin.

3. Role of Vitamin K in respiration:

• Dicumarol and warfarin uncouple oxidative phosphorylation (inhibit oxidative phosphorylation).
• Warfarin and dicumarol are also the inhibitor of Vitamin K activity.
• Vitamin K has ubiquinone like structure.
• This fact suggest that vitamin K has role in cellular respiration.

Recommended Daily Allowance (RDA) of Vitamin K:

• The RDA varies from 70-140 g/day.

Dietary source of Vitamin K:

• Cabbage, cauliflower, spinach, alfa-alfa, tomatoes, are good source of vitamin K.
• Vitamin K is also found in eggs, butter, cheese and dairy products.

Vitamin K Deficiency Syndrome:

• Vit-K is widely distributed in nature.
• Therefore vit-K deficiency is rare occurrence.
•  Vitamin K deficiency can occur due to consumption of antibiotics which with intestinal bacteria.
• Excessive loss of Vit-K as in diarrheal diseases and due to lack of bile salts, for the absorption of Vitamin K from the intestine.
• Vitamin K deficiency increases the coagulation time.
• This is because of the inactivation of blood clotting factors II, VII, IX and X.
• Profuse bleeding occurs in minor injuries.
• In elderly people and especially in malnourished, a condition called hypoprothrombinemia occurs.
• In child and in infant, excess Vitamin K can result in hemolytic anemia and jaundice.
• This is because of its severe effect on RBC.
• In infants, vitamin K should be supplemented early in their lifetime because they have sterile intestine plus mother’s milk cannot provide sufficient Vitamin K.
• People with severe vit K deficiency also require Vit-K supplement.
• Supplements of Vit-K is allowed along with antibiotics as Cefamandole, moxalactam, and loracarbef.

Vitamin K: Structure, Properties, Biological roles and Deficiency

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